In an upright treatment system (described in Applicant's U.S. patent application Ser. No. 11/926,145, filed 29 Oct. 2007, the disclosure of which is incorporated herein by reference), an upright patient rotates about a vertical patient rotation axis and a stationary treatment beam is horizontally oriented toward a target generally positioned at a nominal isocenter, which is the intersection of the horizontal treatment beam with the vertical patient rotation axis.
Angular irradiation with a non-horizontal treatment beam can be achieved by moving the radiation source through a desired angle on an arc centered at the isocenter. If the treatment beam is detected by a downstream detector, the detector should also move accordingly. However, in such a system there is a large source to isocenter distance (about 1 meter), and so the radiation source must be moved through a large elevation angle to achieve a desired angular irradiation. Moving a heavy source-housing on a vertical arc is expensive and cumbersome.